Thanks to everyone for your comments and responses to the week 1 course content. We hope you like the feedback video. Here are some answers to questions that have come up and we didn’t address there…

What is the role of cloud cover – does it provide a cooling effect?

Yes, especially low cloud cover. Clouds reflect sunlight back to space, preventing it reaching the ground where it would have a chance to be absorbed and to heat the Earth. Clouds make the biggest contribution to determining the Earth’s albedo (reflectivity) as seen from space, because at any one time they typically cover about half of the Earth’s surface and they are very white (reflective).

At the same time clouds have a warming ‘blanket effect’ because they trap heat radiation coming up from the Earth and send some of it back down again. This should be familiar if you think about the difference in temperature between a clear night (cold) and a cloudy night (warmer).

So, which effect wins out? Well, it depends on the altitude of the clouds. Low level clouds have a net cooling effect on the Earth, whereas high-altitude clouds have a net warming effect. Globally, the cooling effect wins.

Interestingly the reflectivity of clouds depends on whether the cloud water is arranged in lots of small droplets (highly reflective) or relatively few large droplets (less reflective). Amazingly this property of clouds is partly controlled by biology – tiny algae in the ocean make a gas called dimethyl sulphide which oxidises in the atmosphere to form tiny particles on which cloud droplets form.

Can you give an example of where we can see the Planck feedback in action?

We can see it in action all the time. If I take a hot coal out of the fire it cools down because it radiates away heat – until it reaches the temperature of its surroundings. As day turns to night the ground is no longer heated by the Sun and it starts to cool down by radiating away heat. If I put some food in the fridge (a cold place) it cools down by radiating away heat. Equally if I bring a cold object out of the fridge it starts to warm up because it is giving off less heat than it is gaining from its surroundings.

At the surface of the Earth another really important feedback is the water vapour feedback we met in the video – the loss of ‘latent’ heat caused by evaporating water. There is also the loss of ‘sensible’ heat – that is where heat is transferred to the atmosphere (assuming it is cooler than the surface).

What role is man playing in these naturally balanced feedback cycles?

Human activities are both affecting natural feedback cycles in the climate system and creating some new effects. For example, some of our atmospheric pollution is cooling things down and acting to partially counteract the blanket effect of increasing ‘greehouse’ gases. In particular, burning coal and other fossil fuels produces sulphur dioxide, a gas that oxidises to sulphate which forms tiny particles called ‘aerosol’ in the atmosphere. This aerosol scatters sunlight acting to cool the Earth, and it also forms the nuclei on which cloud droplets can condense – and those cloud droplets also overall act to cool the Earth.

Other human activities are adding to the overall warming. For example, by deforesting the tropics humans are tending to dry out the land surface and reduce cloud cover, and this makes the deforested land heat up faster.

Hopefully this gives just some flavour of the wonderful complexity of the climate system.

Professor Tim